Over recent years, with the popularization of car-interior displays, large-screen TV sets, mobile phones, and lap-top computers, there is an increasing demand for liquid crystal displays (hereinafter also referred to LCDs) which serve in a category of various display devices. LCDs have been widely used as monitors due to their small footprint and low energy consumption features, compared to old-fashioned CRT displays, and have become common in application to TV sets. For these LCDs, various optical films such as polarization films or retardation films are used.
Such an LCD is structured in such a manner that polarization plates are provided on both sides of a liquid crystal cell. A polarization plate passes only light of a polarized wave plane from a predetermined direction. Therefore, the polarization plate plays a significant role in visualizing variations of the orientation of a liquid crystal via an electric field in a liquid crystal display. Namely, performance of the liquid crystal display largely depends on performance of the polarization plate. The polarization plate is commonly structured in such a manner that protective films are laminated on both sides of a polarizer. In some cases, such protective films have a retardation compensation function. LCDs are structured by laminating thus-structured polarization plates to a liquid crystal cell. Protective films are provided to enhance durability of a polarizer. Conventionally, as protective films used for polarization plates, optical films, which are transparent and exhibit excellent physical and mechanical properties, as well as minimal dimensional variation against varying temperature or humidity, have been used.
Recently, with the increasing demand for various display devices, productivity enhancement has been demanded for optical films for use in these devices. In order to increase productivity of optical films, width increase of the optical films and a high-rate production process are needed. Further, to make various display devices thinner, thinner and lighter optical films have been sought. Still further, with realization of larger screen sizes of various display devices, width increase of optical films has also been demanded. Further, for higher productivity, winding of longer films on a single core has been in progress.
Further, to improve mainly mechanical strength of these optical films, and also film characteristics, storage stability, and optical characteristics, it is necessary to add, to the optical films, various additives (e.g., a plasticizer, an antioxidant, a UV absorbent, a matting agent, a conductive substance, an antistatic agent, a flame retardant, and a lubricant).
Conventionally, such optical films have been produced via a solution casting film forming method wherein a dope, prepared by dissolving a resin and various additives in a solvent, is cast on an endless support, and then the solvent is removed in a drying process to wind the film. When optical films are produced via the solution casting film forming method, optical characteristics and flatness thereof may be adjusted by stretching employing a tenter after film formation.
However, with speeding-up and longer film winding for the purpose of thinning, width increase, and productivity enhancement of optical films, damage (e.g., a convex defect of a wound film and sticking during storage), which was not noted in conventional products, has become obvious. Such damage, resulting in defective damage after polarization plate and panel processing, needs to be eliminated. Herein, “convex defect” refers to “embossment” caused when air, having been captured between film layers during winding in film production, remains without coming out, or when foreign materials entering therebetween are generated.
Especially in the case of width increase, the weight of a wound body causes the center portion to bend on its own. Since the load is concentrated in the center portion, it is very probable that a sticking defect over time (during storage) occurs and a convex defect, resulting from embossing of the film due to local film friction, occurs.
With regard to such defects becoming obvious with film thinning, investigations have so far been conducted. For example, there is known a method wherein dynamic friction coefficients of one side of a film and the other side thereof are controlled by adding solid fine particles (for example, refer to Patent Document 1). Further known is a method wherein a cellulose ester film, having been wound on a winding core, is wrapped with a wrapping material of a moisture permeability of 1 g/m2, whereby occurrence of a foreign material defect or a sticking defect of the film itself during long term storage is prevented (for example, refer to Patent Document 2).
In the methods of Patent Documents 1 and 2, effects are exhibited in optical films featuring conventional width. However, these are insufficient countermeasures on the above defects (for example, a convex defect of a wound film and sticking during storage), having become obvious with speeding-up and longer film winding for the purpose of thinning, width increase, and productivity improvement of optical films, which have been pursued over recent years.
In view of such situations, it has been desirable to develop an optical film wherein there is no tendency of occurrence of a sticking defect over time (during storage) in the form of a long wound film of less thickness and increased width, and of occurrence of a convex defect resulting from embossing of the film due to local film friction; a process for producing an optical film; and a polarization plate utilizing the optical film.    Patent Document 1: Unexamined Japanese Patent Application Publication (hereinafter also referred to as JP-A) No. 2004-168981    Patent Document 2: JP-A No. 2005-104556